From 45f1e427fc8b5722b1179eff6375d3da24299ddc Mon Sep 17 00:00:00 2001
From: Tassilo Horn <tsdh@gnu.org>
Date: Sat, 15 Jan 2022 11:23:46 +0100
Subject: [PATCH] mailcap: Docfix & handle function-viewer case in
 mailcap-view-file

* lisp/net/mailcap.el (mailcap-mime-data): Fix docstring: if viewer is
a symbol/function, it should have zero args and expect the file's
contents to be in the current buffer.
(mailcap-view-file): Implement the case where the chosen viewer is a
function and not a shell command.
---
 lisp/net/mailcap.el | 72 ++++++++++++++++++++++++++-------------------
 1 file changed, 42 insertions(+), 30 deletions(-)

diff --git a/lisp/net/mailcap.el b/lisp/net/mailcap.el
index daa2d5a3fb3..b65f7c25b83 100644
--- a/lisp/net/mailcap.el
+++ b/lisp/net/mailcap.el
@@ -319,8 +319,9 @@ attribute name (viewer, test, etc).  This looks like:
 
 Where VIEWERINFO specifies how the content-type is viewed.  Can be
 a string, in which case it is run through a shell, with appropriate
-parameters, or a symbol, in which case the symbol is `funcall'ed if
-and only if it exists as a function, with the buffer as an argument.
+parameters, or a symbol, in which case the symbol must name a function
+of zero arguments which is called in a buffer holding the MIME part's
+content.
 
 TESTINFO is a test for the viewer's applicability, or nil.  If nil, it
 means the viewer is always valid.  If it is a Lisp function, it is
@@ -1175,34 +1176,45 @@ See \"~/.mailcap\", `mailcap-mime-data' and related files and variables."
   (mailcap-parse-mailcaps)
   (let ((command (mailcap-mime-info
                   (mailcap-extension-to-mime (file-name-extension file)))))
-    (unless command
-      (error "No viewer for %s" (file-name-extension file)))
-    ;; Remove quotes around the file name - we'll use shell-quote-argument.
-    (while (string-match "['\"]%s['\"]" command)
-      (setq command (replace-match "%s" t t command)))
-    (setq command (replace-regexp-in-string
-		   "%s"
-		   (shell-quote-argument (convert-standard-filename file))
-		   command
-		   nil t))
-    ;; Handlers such as "gio open" and kde-open5 start viewer in background
-    ;; and exit immediately.  Avoid `start-process' since it assumes
-    ;; :connection-type `pty' and kills children processes with SIGHUP
-    ;; when temporary terminal session is finished (Bug#44824).
-    ;; An alternative is `process-connection-type' let-bound to nil for
-    ;; `start-process-shell-command' call (with no chance to report failure).
-    (make-process
-     :name "mailcap-view-file"
-     :connection-type 'pipe
-     :buffer nil ; "*Messages*" may be suitable for debugging
-     :sentinel (lambda (proc event)
-                 (when (and (memq (process-status proc) '(exit signal))
-                            (/= (process-exit-status proc) 0))
-                   (message
-                    "Command %s: %s."
-                    (mapconcat #'identity (process-command proc) " ")
-                    (substring event 0 -1))))
-     :command (list shell-file-name shell-command-switch command))))
+    (if (functionp command)
+        ;; command is a viewer function (a mode) expecting the file
+        ;; contents to be in the current buffer.
+        (let ((buf (generate-new-buffer (file-name-nondirectory file))))
+          (set-buffer buf)
+          (insert-file-contents file)
+          (setq buffer-file-name file)
+          (funcall command)
+          (set-buffer-modified-p nil)
+          (pop-to-buffer buf))
+      ;; command is a program to run with file as an argument.
+      (unless command
+        (error "No viewer for %s" (file-name-extension file)))
+      ;; Remove quotes around the file name - we'll use shell-quote-argument.
+      (while (string-match "['\"]%s['\"]" command)
+        (setq command (replace-match "%s" t t command)))
+      (setq command (replace-regexp-in-string
+		     "%s"
+		     (shell-quote-argument (convert-standard-filename file))
+		     command
+		     nil t))
+      ;; Handlers such as "gio open" and kde-open5 start viewer in background
+      ;; and exit immediately.  Avoid `start-process' since it assumes
+      ;; :connection-type `pty' and kills children processes with SIGHUP
+      ;; when temporary terminal session is finished (Bug#44824).
+      ;; An alternative is `process-connection-type' let-bound to nil for
+      ;; `start-process-shell-command' call (with no chance to report failure).
+      (make-process
+       :name "mailcap-view-file"
+       :connection-type 'pipe
+       :buffer nil ; "*Messages*" may be suitable for debugging
+       :sentinel (lambda (proc event)
+                   (when (and (memq (process-status proc) '(exit signal))
+                              (/= (process-exit-status proc) 0))
+                     (message
+                      "Command %s: %s."
+                      (mapconcat #'identity (process-command proc) " ")
+                      (substring event 0 -1))))
+       :command (list shell-file-name shell-command-switch command)))))
 
 (provide 'mailcap)
 
-- 
2.39.2